Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a transmission line probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.
An electromagnetic transmit signal is generated by a transceiver and propagated towards the surface of the product in the tank, and an electromagnetic reflection signal resulting from reflection of the transmit signal at the surface is propagated back towards to the transceiver.
Based on a relation between the transmit signal and the reflection signal, the distance to the surface of the product can be determined.
Radar level gauge systems are ubiquitous in application areas involving handling, shipping and storing of products as well as, for example, in the chemical process industry.
Since products to be monitored and/or measured are often flammable, special safety requirements exist for equipment, such as radar level gauge systems or at least parts thereof that are positioned within a so-called hazardous area. Such equipment generally needs to be certified as either explosion-proof or intrinsically safe.
For instance, the radar level gauge system may need to be designed to ensure that an explosion inside the housing for the measurement electronics does not result in ignition of flammable material inside the tank. To that end, a feed-through connecting the measurement electronics arranged outside the tank with an antenna in the interior of the tank may need to fulfill certain requirements, such as those detailed by international standard IEC 60079-1 or similar standards.
U.S. Pat. No. 8,763,453 discloses an arrangement for measuring a fill level of a fill substance in a container, with a gastight feedthrough between the outside and the inside of the container.
According to U.S. Pat. No. 8,763,453, the gas tight feedthrough comprises a hollow conductor with a microwave transparent window inserted gas tightly into the hollow conductor. The window includes a planar disk that has a thickness corresponding approximately to a half wavelength or a small integer multiple times the half wavelength of the microwave signals at a predetermined signal frequency in the disk. In order to increase the bandwidth over which the transition through the feedthrough is as low reflection as possible, a matching layer is provided on the two oppositely lying outer surfaces of the disk. Each matching layer has a thickness corresponding to a fourth of the wavelength of the microwave signals at the predetermined signal frequency in the matching layer. According to U.S. Pat. No. 8,763,453, each matching layer preferably has a dielectric constant equaling a square root of the product of the dielectric constant of the disk and the dielectric constant of the hollow conductor above or below the disk.
U.S. Pat. No. 8,763,453 further discloses that, in applications in which only a single microwave mode is to be sent and received, the feedthrough with rectangular cross-section is combined with an antenna with a circular disk shaped cross section, with a transition element arranged between the feedthrough and the antenna.
The arrangement according to U.S. Pat. No. 8,763,453 appears to be rather complicated and relatively costly.